FIG. 2 is a developed view for showing the internal structure of element 11 (not shown) of this type of conventional capacitor. Element 11 has the following structure. That is, a pair of electrodes (positive and negative) have polarizable electrode layer 25 formed on collector 27 made of aluminum foil. The electrodes are displaced in the reverse direction to each other, separator 26 is interposed between them, and they are rolled up. A part of collector 27 where polarizable electrode layer 25 is not formed is an unformed part of the polarizable electrode layer, and part of it becomes positive electrode 21b and negative electrode 21c. FIG. 3 is a sectional view showing the structure of a conventional capacitor. In FIG. 3, element 11 has hollow 11a. From both end surfaces of element 11, positive and negative electrodes are extracted respectively. Element 11, together with a driving electrolyte (not shown), is contained in based, cylindrical metal case 12 made of aluminum. Metal case 12 has projection 12a provided integrally on the inner bottom surface of metal case 12 so as to fit into hollow 11a of element 11. This projection 12a is fitted into hollow 11a of element 11 and is inserted into metal case 12. Then, the end face of element 11 at the negative electrode is joined to the inner bottom surface of metal case 12 by laser welding mechanically and electrically. Terminal plate 13 made of aluminum is joined to the end face of element 11 at the positive electrode and disposed at the opening of metal case 12 for sealing.
FIGS. 4A, 4B, 4C, and 4D are a perspective view showing the structure of terminal plate 13 used for a conventional capacitor, viewed from an outer surface side; a perspective view of the same, viewed from an inner surface side; a sectional view taken along 4C-4C in FIG. 4B; and a sectional view taken along 4D-4D in FIG. 4B, respectively. This terminal plate 13 has the following structure. That is, the inner surface of element 11 where the end face at the positive electrode is joined is base level D (refer to FIG. 4D). The base level, except for outer circumference 13a and plural joint parts 13b strip-shaped running from this outer circumference 13a toward the center, is upraised toward the outer surface. To assemble the capacitor, joint part 13b is joined to the end face of element 11 at the positive electrode by laser welding mechanically and electrically.
The center of the inner surface of this terminal plate 13 is provided therein with projection 13c fitting into hollow 11a of element 11. Further, the outer surface of this terminal plate 13 is provided thereon with anode terminal 13d for connecting to the outside. Additionally, step 13e for inserting sealing rubber 15 (described later) is provided on the outer circumference at the outer surface circularly, and projection 13f is provided substantially in the center of this step 13e circularly, respectively. Terminal plate 13 further includes antirotation part 13g placed at the outer surface and safety valve mount hole 13h (the safety valve is not shown) doubling as an electrolyte inlet.
A capacitor is produced in the following way. That is, in FIG. 3, projection 13c provided on the inner surface of terminal plate 13 is inserted into hollow 11a of element 11, and then the end face of element 11 at the positive electrode is joined to joint part 13b by laser welding. This terminal plate 13 is disposed at the opening of metal case 12 with insulating member 14 interposed. With sealing rubber 15 arranged on the circumference of the outer surface of terminal plate 13, the open end of metal case 12 undergoes curling so as to pressure-weld sealing rubber 15.
In a conventional capacitor thus produced, the base level of terminal plate 13 corresponds to the joint part joined to the end face of element 11 at the positive electrode, which allows the distance from the end face of element 11 at the positive electrode to the top end of metal case 12 after being processed to be extremely short. As a result, the height of element 11 can be made higher than other capacitors with the same height, thereby increasing capacitance and decreasing resistance.
As information on prior art documents related to the invention of the application, patent literature 1 for example is known.
In a conventional capacitor, the end faces of element 11 at the positive and negative electrodes are joined to the inner surface of terminal plate 13 and the inner bottom surface of metal case 12, respectively, by laser welding. The joint part is joined by laser welding in a state where the end face at the positive electrode is partially crushed by pressing plural strip-shaped joint parts 13b provided on the inner surface of terminal plate 13 (although not shown, the inner bottom surface of metal case 12 is formed similarly) against the end face of element 11 at the positive electrode.
A concrete production method is as the following. That is, element 11 is inserted into metal case 12. Subsequently, terminal plate 13 is arranged at the opening of metal case 12. By pressurizing this terminal plate 13, a joint part provided on the inner bottom surface of metal case 12 partially crushes the end face of element 11 at the negative electrode, and joint part 13b provided on the inner surface of terminal plate 13 partially crushes the end face of element 11 at the positive electrode. After that, laser welding is performed.
Accordingly, the quality of laser welding is susceptible to the extent to which the end faces of element 11 at the positive and negative electrodes are crushed in a desired shape and desired dimensions. However, the state where the end faces of element 11 at the positive and negative electrodes are crushed cannot be monitored from their appearance, which means the joining state after laser welding and its strength cannot be controlled. As a result, a poor crushing state of the end faces of element 11 at the positive and negative electrodes causes variation in joining by laser welding, which results in insufficient welding points to increase the resistance of extracting an electrode and of connection. Consequently, when a charge-discharge load is exerted on a capacitor, the product generates a lot of heat, thus decreasing the product life.    [Patent literature 1] Japanese Patent Unexamined Publication No. 2006-173440